Reactions of phosphines with acetylenes. Part XVI. Formation of β-alkoxyphosphonium ylides and vinyl ethers via methanolysis of vinylphosphonium salts

Abstract

Triphenylphosphine in alcohol reacts (a) with dimethyl acetylenedicarboxylate to give a stabilised β-alkoxy-ylide, (b) with methyl phenylpropiolate to give (Z)-methyl 2-methoxycinnamate, and (c) with methyl propiolate to give a mixture of three isomeric methyl methoxyacrylates. The reactions proceed via nucleophilic attack of the phosphine on the acetylene and then of methoxide on the intermediate vinylphosphonium salt by the pathway which involves least build up of negative charge. Methoxide attacks at the β-carbon atom when the vinyl α-carbon atom which bears the phosphonium group possesses an additional stabilising group, e.g. methoxycarbonyl as in (a), or phenyl as in (b). Methoxide attacks both α- and β-positions when there are balancing α- and β-stabilising groups as in (c). The reactions (b) and (c) involve two trans-addition steps followed by a trans-elimination step.